Recently, a variety of electric equipments such as a mobile phone, a personal computer, an electric home appliance, and so on, has been widely developed. It has been studied to make such electric equipment more compact, to have higher functional capability and lower power consumption. The electric equipment commonly includes a switching circuit between a power supply and a load to have a margin of safety.
Further, the electric equipment may include more complex circuit which may need a multiple power supplying system in order for the electric equipment to have a high performance. The multiple power supplying system supplies multiple voltages and a variety of power capacities. Namely, it has become more important to control the multiple power supplying system by switching each power supply so as to obtain a safe equipment having a low power consumption, and avoid faulty operation.
Thus, a switching circuit is generally employed to protect electric equipment from damages due to overheat and protect a power supply, such as battery, etc., from a reverse current flowing from a load to the power supply. It is desirable for the switching circuit to have less power consumption, because the switching circuit is an auxiliary circuit for the electric equipment. Such a switching circuit is called a backflow prevention circuit.
FIG. 1 illustrates an example of a conventional backflow prevention circuit 100. The backflow prevention circuit 100 includes a MOS (metal oxide semiconductor) switch 103, a diode 109 and a gate drive circuit 110. The diode 109 and the MOS switch 103 are connected in series between a power supply 107 and a load 108.
When a voltage of the power supply 107 at an input terminal IN is larger than a voltage of a cathode of the diode 109 at an output terminal OUT, a current flows through the diode 109 because a forward voltage is applied on the diode 109. Meanwhile, when the voltage of the cathode of the diode 109 at the output terminal OUT is larger than the voltage of power supply 107 at the input terminal IN, the current through the diode 109 is restricted and may be a small reverse current which is defined by a current-to-voltage characteristic of the diode 109.
In the backflow prevention circuit 100 of FIG. 1, however, there is a power loss at the diode 109 due to the forward current of the diode 109. Further, a resistance between the power supply 107 and the load 108 is increased due to a resistance of the diode 109 in addition to a resistance of the MOS switch 103.
FIG. 2 illustrates another example of the conventional backflow prevention circuit 200. The backflow prevention circuit 200 includes a MOS switch 113, a comparator 111, a bias voltage source 112 and a gate drive circuit 120. The bias voltage source 112 is employed to improve a noise margin of the backflow prevention circuit 200 and is connected between the power supply 107 and an inverted-input terminal of the comparator 111. The MOS switch 113 is formed of a P-MOS (p-channel metal oxide semiconductor) transistor. A voltage of the bias voltage source 112 is set to be smaller than the voltage of the power supply 107.
The comparator 111 compares voltages between an output voltage at the output terminal OUT and the voltage of the power supply 107 at the input terminal IN. When the output voltage becomes lower than a predetermined voltage Vc, the comparator 111 outputs a high level signal to a gate of the P-MOS transistor (MOS switch 113) so as to fix the MOS switch 113 to be off. The predetermined voltage Vc is defined by the following formula:Vc=Vp−Vb where Vp is the voltage of the power supply 107 and Vb is the bias voltage source 112. Thus, a reverse current is prevented by setting the MOS switch 113 to be in shutdown state. The MOS switch 113 is off in a shutdown state.
In the backflow prevention circuit 200 of FIG. 2, however, minimum voltage for the operation is relatively high because the comparator 111 may be formed of a differential amplifier which requires a larger voltage from the power supply to operate. Further, there may be a penalty in power consumption because the differential amplifier may include a constant current source which generates current constantly. Moreover, an operational voltage range may be narrower in comparison to other backflow prevention circuits, because an input voltage for the comparator 111 is restricted to be within a narrower voltage range.
Furthermore, a full voltage range from the power supply voltage to ground voltage may be needed to be input to the comparator 111 of FIG. 2. Therefore, it may be necessary to employ so called Rail-to-Rail input circuit at an input part of the differential amplifier. The so called Rail-to-Rail outputs almost full voltage range from the power supply voltage to ground to the differential amplifier.
However, such input circuit may require approximately twice more circuit elements in comparison to conventional input circuit of the differential amplifier. As a result, the backflow prevention circuit 200 may be a large circuit in size and may have a larger power consumption in comparison with other backflow prevention circuits.